1. Field of the Invention
The present invention relates to a circularly polarized antenna apparatus and a radio communication apparatus including the same.
2. Description of the Related Art
Recent radio communication apparatuses using circularly polarized waves, such as GPS (Global Positioning System) or DAB (Digital Audio Broadcast) systems, for use in mobile vehicles such as automobiles and ships, have incorporated a compact circularly polarized antenna apparatus, as described in Japanese Unexamined Patent Application Publication No. 2000-183637. This type of antenna apparatus is shown in FIG. 10.
In the antenna apparatus shown in FIG. 10, a rectangular radiating electrode 4 having degeneracy-splitting elements 3 is provided on a first principal surface 2 of a solid rectangular substrate 1, and a ground electrode (not shown) is provided on a second principal surface 5 of the substrate 1. A strip feeding electrode 7 is provided on a side surface 6 of the substrate 1 so as to extend to the first principal surface 2 from the second principal surface 5 of the substrate 1. Wide capacitor electrodes 8, which are connected to the ground electrode, are provided at both sides of the feeding electrode 7. These components define a more compact antenna apparatus.
In this antenna apparatus, the length of each edge of the radiating electrode 4 is equal to one half the effective wavelength xcex, that is, xcex/2, of an electromagnetic wave to be radiated. The leading edge of the feeding electrode 7 wraps around to the first principal surface 2 so as to face the center portion of one edge of the radiating electrode 4 with a gap therebetween, such that the feeding electrode 7 is capacitively coupled with the radiating electrode 4. The degeneracy-splitting elements 3 are formed by cutting out the opposing corners of the radiating electrode 4 along a diagonal such that there is a difference between the diagonal electrical lengths of the radiating electrode 4.
With this structure, when signal power is supplied to the feeding electrode 7, two resonant currents which are out of phase by 90xc2x0 are excited along the perpendicular diagonals of the radiating electrode 4. The two resonant currents provide excitation sources from which two spatially perpendicular electromagnetic waves having different frequencies radiate in a direction that is perpendicular to the feeding electrode 7.
In the aforementioned antenna apparatus, in order to reduce the dimensions, the capacitance of the capacitor electrode 8 is increased, while the area of the radiating electrode 4 provided on the first principal surface 2 is reduced. As a result, inevitably, the two resonant currents excited in the radiating electrode 4 flow in the radiating electrode 4 having a small area. Thus, even if the signal power supplied to the feeding electrode 7 increases to provide a high electric field strength for the electromagnetic waves to be radiated, the conductor loss of the radiating electrode 4 increases, which leads to a decrease in antenna gain.
In order to overcome the problems described above, preferred embodiments of the present invention provide a compact circularly polarized antenna apparatus which achieves high antenna gain, and a radio communication apparatus including the novel circularly polarized antenna apparatus.
One preferred embodiment of the present invention provides a circularly polarized antenna apparatus including a dielectric or magnetic substrate having a first principal surface, a second principal surface, and side surfaces, a radiating electrode provided on the substrate, a ground electrode provided on the second principal surface of the substrate, a feeding element for feeding excitation power to the radiating electrode, and a degeneracy-splitting element which causes two resonant currents to be excited in the radiating electrode, the two resonant current being split between degenerate modes. The radiating electrode is defined by a primary radiating electrode and secondary radiating electrodes, the primary radiating electrode is provided on the first principal surface of the substrate, and the secondary radiating electrodes are provided on the side surfaces of the substrate so as to connect to the primary radiating electrode, each secondary radiating electrode having substantially the same width as the primary radiating electrode.
The radiating electrode extends from the first principal surface to the side surfaces, and the area of the radiating electrode is increased at least by the area of the secondary radiating electrodes compared to the case where the radiating electrode is provided only on the first principal surface. This elongates the paths of the two resonant currents excited in the radiating electrode, thereby reducing the conductor loss of the primary radiating electrode. Moreover, since the area of the radiating electrode is increased, the size of the substrate is reduced, thus providing a compact antenna apparatus.
The degeneracy-splitting elements allow a difference between the electrical lengths along diagonals of the radiating electrode including the secondary radiating electrodes, thus causing two resonant currents to be excited along diagonals of the radiating electrode when signal power is supplied to the radiating electrode from the feeding element. The length of each edge of the radiating electrode preferably is substantially one half the effective wavelength of an electromagnetic wave to be radiated, although the secondary radiating electrodes are provided on the side surfaces of the substrate. This causes the two resonant currents which are out of phase with each other by 90xc2x0 to flow in a substantially perpendicular manner.
Accordingly, the radiating electrode is provided on a first principal surface and on side surfaces of the substrate, thus achieving a compact circularly polarized antenna apparatus having a greatly reduced conductor loss of the primary radiating electrode with an increase in antenna gain.
The substrate preferably has a substantially rectangular shape with a first principal surface, a second principal surface, and four side surfaces. The primary radiating electrode of the radiating electrode is provided on the first principal surface of the substrate, and the secondary radiating electrodes of the radiating electrode are provided on two opposing side surfaces of the substrate.
The radiating electrode extends from the first principal surface to the two side surfaces, and the area of the radiating electrode is increased by the area of the secondary radiating electrodes provided on the two side surfaces in addition to the primary radiating electrode provided on the first principal surface. This elongates the diagonal electrical paths from the corners of one side surface to the corners of the other side surface, thereby reducing the conductor loss of the primary radiating electrode from which electromagnetic waves primarily radiate. Moreover, while the secondary radiating electrodes are provided on two side surfaces of the substrate, no secondary radiating electrode is provided on the other side surfaces of the substrate, such that there is no influence on the electric field strength of an electromagnetic wave to be radiated.
Accordingly, the radiating electrode is provided on a first principal surface and on two opposing side surfaces of the substrate, thus increasing the length of the paths of the two resonant currents excited in the radiating electrode. The secondary radiating electrodes are provided only on the two opposing side surfaces, thus achieving a compact circularly polarized antenna apparatus without interfering with the radiation of electromagnetic waves.
The degeneracy-splitting element preferably includes two capacitor electrodes having different lengths on the side surface of the substrate on which each of the secondary radiating electrodes is provided, each capacitor electrode having one end connected to the ground electrode, the capacitor electrodes extending towards the corners of each secondary radiating electrode.
Since the gaps differ between the capacitor electrodes and the secondary radiating electrodes, the secondary radiating electrodes and the capacitor electrodes are capacitively coupled via the capacitance having different capacitance values, thus causing two resonant currents split between the degenerate modes to be excited in the radiating electrode. If the capacitor electrodes are provided on two opposing side surfaces on which the secondary radiating electrodes are provided, the capacitor electrodes in a diagonally opposing state with respect to the radiating electrode have the same length, thereby reliably achieving the split modes.
The capacitance on the side surfaces of the substrate on which the capacitor electrodes are provided generates electrical paths in which the resonant currents flow along diagonals of the radiating electrode. The resonant currents flow in the primary radiating electrode and the secondary radiating electrodes, that is, the paths of the resonant currents flowing in the radiating electrode are elongated, thereby reducing the conductor loss of the primary radiating electrode.
Accordingly, the secondary radiating electrodes and the capacitor electrodes are provided on the side surfaces of the substrate, thus causing resonant currents in the degeneracy-split modes to be excited in the radiating electrode. Therefore, the resonance conditions are adjusted depending upon the capacitance therebetween. The two resonant currents flow in the direction toward the capacitor electrodes, thus increasing the lengths of the paths of the resonant currents flowing in the radiating electrode.
The degeneracy-splitting element is preferably formed by cutting out the corners of the secondary radiating electrodes along a diagonal of the radiating electrode.
With this structure, the degeneracy-splitting element is defined depending upon the secondary radiating electrodes provided on the side surfaces of the substrate. Thus, two resonant currents having different frequencies are excited in the radiating electrode without a change in the area of the primary radiating electrode from which electromagnetic waves primarily radiate. Again, this allows the resonant currents to flow in the primary radiating electrode and the secondary radiating electrodes, thus reducing the conductor loss of the primary radiating electrode.
Accordingly, the degeneracy-splitting element is provided in the secondary radiating electrodes on the side surfaces of the substrate, thus not requiring a reduction in the area of the primary radiating electrode from which electromagnetic waves primarily radiate. Thus, the antenna gain is greater than that in an antenna apparatus in the related art having a radiating electrode provided on a first principal surface of the substrate.
The primary radiating electrode of the radiating electrode is preferably notched at both side edges thereof which extend to the secondary radiating electrodes.
Thus, the electrical lengths of the radiating electrode in the direction extending to the secondary radiating electrodes are increased. The diagonal electrical lengths of the radiating electrode vary depending upon the depth of the notched portions or the number of notched portions. Therefore, the resonant frequencies of the two resonant currents in the degeneracy-split modes can be readily adjusted by forming the notched portions as appropriate. The angle of the two resonant currents in the split modes can also be adjusted.
The notched portions increase the diagonal electrical lengths of the radiating electrode. In consideration of the electrical lengths, the capacitance between the secondary radiating electrodes and the capacitor electrodes can be reduced. The secondary radiating electrodes and the capacitor electrodes are printed with a large tolerance for printing variations, thus increasing the production yield of circular polarized antenna apparatuses.
The primary radiating electrode preferably includes a slit extending along a diagonal of the radiating electrode. This increases the diagonal electrical length of the radiating electrode in the longitudinal direction of the slit to greater than that in the direction that is perpendicular to the longitudinal direction of the slit. The electrical length in the direction that is perpendicular to the longitudinal direction of the slit can be adjusted by changing the length of the slit, such that the difference in frequency between the two resonant currents is adjusted. With the slit and the capacitor electrodes, two resonant currents are reliably split between the degenerate modes in the radiating electrode. With this structure, again, the capacitance between the secondary radiating electrodes and the capacitor electrodes is greatly reduced.
The feeding element is preferably a strip feeding electrode provided on one side surface of the substrate so as to extend from the second principal surface of the substrate towards the edge of one of the secondary radiating electrodes.
The leading edge of the feeding electrode is capacitively coupled with the edge of the secondary radiating electrode, or alternatively, may be directly connected to the edge of the secondary radiating electrode, thus providing a simplified structure. The feeding electrode can be printed together with the secondary radiating electrodes and the capacitor electrodes, thus reducing the number of production steps for the circularly polarized antenna apparatus. Furthermore, the feeding electrode can be directly provided on the substrate, such that the circularly polarized antenna apparatus is mounted on a circuit board in a radio communication apparatus using a surface mount technique.
The feeding element is preferably a feed line which is inserted through the substrate from the second principal surface and which is isolated from the ground electrode.
Accordingly, the radiating electrode is directly fed through a feed line inserted through the substrate, such that the feed point provides impedance matching between the radiating electrode and the feed line, thereby achieving an efficient supply of signal power. This requires no impedance matching circuit, and thus the feeding circuit structure is simplified.
In another preferred embodiment of the present invention, a radio communication apparatus includes a circuit board having a radio-frequency receiving circuit or a radio-frequency transmitting and receiving circuit. The circularly polarized antenna apparatus including any of the structures according to preferred embodiments described above is mounted on the circuit board, in which the feeding element is connected to the input terminal of the receiving circuit or the transmitting and receiving circuit.
The radio communication apparatus including a compact circularly polarized antenna apparatus having high antenna gain is capable of more remote communications with the same transmission power, and is more sensitive to reception signals to receive weaker radio waves than a radio communication apparatus in the related art. The radio communication apparatus including such a highly compact circularly polarized antenna apparatus is very compact.
Other feature, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention with reference to the attached drawings.